Site-specific post-translational modifications of histones that include methylation, acetylation, phosphorylation and ubiquitination function individually and/or combinatorially as fundamental epigenetic mechanisms in histone-directed chromatin biology that governs all nuclear DNA-templated processes in eukaryotic organisms. Histone lysine methylation is almost exclusively catalyzed by a large family of the SET domain histone lysine methyltransferases (HKMTs), some of which have been implicated in human diseases including cancers and leukemia. Histone lysine methylation is generally more complex than lysine acetylation, as a lysine can be subjected to mono-, di- or tri-methylation, which have been distinctively linked to different functional consequences in the biological context. However, the catalytic mechanism and substrate specificity of these histone modifying enzymes are still not well understood. Notably, within this extensive protein family there is a subclass of SET domain proteins that are encoded by viruses as well as bacteria, including Bacillus anthraces that are a known causative agent for anthrax in humans, but little is known about their cellular functions. Our current study suggests that the SET domain proteins from the chlorella viruses possibly play a role in silencing host gene transcription by methylating host histone H3 at lysine 27, a modification known to trigger long-term gene silencing in eukaryotic cells. These pathogen SET domain HKMTs offer excellent model systems to characterize the catalysis and substrate specificity for the large family of SET domain HKMTs, as well as to investigate possibly novel mechanisms by which viruses or bacteria use to suppress host gene transcription. In this Project, we plan to perform structure-based molecular and cellular biology analyses of a group of SET domain proteins from chlorella viruses and Bacillus anthracis. We specifically aim to address the fundamental questions of the molecular underpinnings concerning the catalytic mechanism, methylation multiplicity and site specificity of this group of pathogen SET domain HKMTs. While viruses are known to recruit cellular proteins for viral genome maintenance and replication, whether and how viruses directly modify host histones to interfere host gene transcription is not known. Therefore, these planned studies offer us with a unique opportunity to investigate host-pathogen interactions at host chromatin level, and to gain new insights into the life cycle of these pathogens that may have implications in the molecular pathogenesis in the related human diseases.